docs: def assert_is_none(expr, msg_fmt="{msg}"):
      """Fail if the expression is not None.
  
      >>> assert_is_none(None)
      >>> assert_is_none(False)
      Traceback (most recent call last):
          ...
      AssertionError: False is not None
  
      The following msg_fmt arguments are supported:
      * msg - the default error message
      * expr - tested expression
      """
  
      if expr is not None:
          msg = "{!r} is not None".format(expr)
          fail(msg_fmt.format(msg=msg, expr=expr))